1. Field of the Invention
The present invention relates to a display device having a plastic substrate and a fabrication method therefor, and more particularly, relates to a display device having a fiber-filled plastic substrate.
2. Description of the Related Art
In recent years, use of liquid crystal display devices and organic EL display devices for portable information terminal equipment has expanded thanks to their features of being thin, light in weight and low in power consumption. With this expansion of the use, there have arisen strong requests that such display devices should be further light and thin and improved in shock resistance and other properties.
To respond to the above requests, an idea of using a plastic substrate instead of the conventionally used glass substrate has been proposed. However, many problems to be overcome are present to adopt a plastic substrate made of a resin material in a sheet shape as a substrate of a display device.
One of the biggest problems is that a plastic substrate is large in linear expansivity (i.e., coefficient of linear thermal expansion). While the linear expansivity of glass is generally several ppm/° C., that of even a type of plastic small in linear expansivity is as large as several tens of ppm/° C. Being large in linear expansivity indicates that the variation in size with temperature is large. Using such a material, therefore, it is difficult to fabricate drive elements, such as TFTs, which require high-precision patterning. A plastic substrate may be used as a counter substrate while the conventional glass substrate being used as the substrate on which TFTs are formed (hereinafter, this substrate is also simply called a “TFT substrate”). In this case, also, difficulty is found in positioning color filters (and/or a black matrix) formed on the counter substrate with respect to pixel electrodes on the TFT substrate.
To reduce the linear expansivity of a plastic substrate to thereby improve the size stability, there have been proposed methods of forming a plastic substrate using a material having a filler mixed in a resin matrix (composite material). As used herein, a plastic substrate formed of a composite material is specifically called a “composite substrate” in some cases.
For example, Japanese Laid-Open Patent Publication No. 11-2812 (Literature 1) discloses a reflection conductive substrate having a composite substrate formed by impregnating a glass fiber fabric with a resin and curing the resin.
Japanese Laid-Open Patent Publication No. 2001-133761 (Literature 2) discloses a composite substrate having fibers arranged in lines or stripes in a resin so as to be kept off from one another. According to Literature 2, the composite substrate having the impregnated fiber fabric (woven fabric) disclosed in Literature 1 has a problem that woven portions and intersecting portions of fibers of the fiber fabric cause development of minute unevenness on the substrate surface and this degrades the display quality. Literature 2 argues that the disclosed arrangement can provide a composite substrate having a flat surface.
As pointed out in Literature 2, a plastic substrate formed using a glass fiber fabric has an uneven surface. As for the plastic substrate disclosed in Literature 2, fabrication of such a plastic substrate is difficult. Even if the fabrication is successful, it is difficult to reduce the surface unevenness to the level of 100 nm or less, for example.
To planarize the surface of the plastic substrate, a planarizing film may be formed on the uneven surface described above. By forming such a planarizing film, a flat surface having unevenness reduced to the level of 100 nm or less can be provided. However, according to examinations conducted by the inventors of the present invention, the following problem occurs. When an inorganic barrier layer is formed on the planarized surface, unevenness of 100 nm or more develops on the surface of the resultant inorganic barrier layer in some cases. The inorganic barrier layer is formed to improve the barrier properties of the plastic substrate against water and/or oxygen in the air.